Situps are widely recognized as a very beneficial exercise for firming up the stomach and abdominal muscles, for health and cosmetic purposes, as well as for personal satisfaction.
In the situp, the exerciser lies on a supporting surface on his backside and elevates the upper body including the arms and head off the supporting surface. As the body pivots generally about the buttock which remains against the supporting surface, the natural tendency is for the feet to lift up, somewhat at least, off the floor. The exercise can be performed much more rapidly and more beneficially if someone, or a device of some type, holds the feet firmly in place relative to the supporting surface. This also allows for doing situps with weights carried by the exerciser to even further increase the force needed to lift the upper body off the supporting surface.
Exercise clubs frequently have situp boards for holding the feet down to assist one in doing situp exercises. One type board uses a well padded bar located some 6 to 8 inches off the supporting board surface which allows the user to lock his feet and/or ankle under the supporting bar. Another device uses a flexible strap that can be put over the foot and ankle area. However, most people do not belong or have access to that type club, or otherwise do not have such equipment for home use.